As is known in the art, a loudspeaker type telephone instrument has a microphone for transmitting voice signals and a loudspeaker for receiving voice signals transmitted from the microphone of another loudspeaker type telephone instrument.
The loudspeaker is capable of emitting a relatively large amount of sound energy, much more than the amount of sound energy emitted by an earphone of a conventional handheld telephone instrument. The loudspeaker and the microphone of a loudspeaker or hand-free type of telephone instrument are assembled in a relatively close relationship within the same casing or cabinet of the instrument. Inevitably, the loudspeaker is acoustically coupled with the microphone in a strong manner. An interphone instrument also has a pair of electric/acoustic and acoustic/electric converters which are acoustically coupled with each other.
The acoustic coupling between a pair of converters (loudspeaker and microphone) of a hand-free telephone instrument and/or interphone apparatus is apt to produce a number of adverse effects on the bidirectional voice-communication system with a pair of hand-free telephone or interphone instruments.
For example, assuming that a first microphone of a first hand-free telephone instrument is connected via a voice transmission line or channel to a second loudspeaker of second hand-free telephone instrument and that a second microphone of the second telephone instrument is connected via another voice transmission line to a first loudspeaker of the first telephone instrument, an acoustic energy inputted to the first microphone is fed back to itself, because a closed loop is formed owing to the acoustic coupling between the loudspeaker and the corresponding microphone. The level of the fed-back signal often becomes high enough to produce a howling.
With a view that a relatively strong acoustic coupling between the loudspeaker and the microphone is inevitable, one and probably the only technique to completely eliminate the occurrence of howling is to provide control means for disconnecting or substantially disconnecting one or more voice transmission channels so that no closed loop is formed during operation, thus making a selected voice transmission channel which is required to transmit a voice message.
Such a control means or system is called "voice switching control system" which is the subject matter of the present invention.
A typical prior voice switching control system includes a pair of voice switches or variolossers. One of the voice switches is disposed in a first voice transmission line or channel between a first microphone of the first telephone or interphone instrument or station and a second loudspeaker of the second station. The other of the voice switches is disposed in a second voice transmission line between a second microphone of the second station and a first loudspeaker of the first station. In operation, a pair of voice transmission lines is energized in a complementary manner by the switching of the two voice switches which introduces two different attenuation (loss) factors in voice signals from associated microphones. The operation mode ("ON or OFF state") of the voice switches is controlled by a control circuit which monitors a pair of outputs from the first and second microphones, i.e.--the voice signal conditions in the first and second voice transmission lines ahead of the voice switches. The two outputs are passed through wave shaping circuits to be compared by a comparator. Whenever the level of the first microphone output voice signal in the first voice transmission line is greater than the level of the second microphone output voice signal in the second voice transmission line, the comparator controls the first voice switch to remain "ON" and controls the second voice switch to remain "OFF", and vice versa. Accordingly, no closed loop is formed during operation. However, owing to the strong acoustic-loudspeaker/microphone-coupling and since it is necessary to reproduce a high level of sound at a loudspeaker, the level of output from the microphone of the station which is receiving a voice message from other station becomes comparable to and sometimes greater than the level of output from the other microphone of the other station. If this occurs, a message cannot be transmitted. Therefore, the control circuit usually includes a pair of auxiliary variolossers, each of which is disposed in the connecting line between an associated voice signal (output of microphone) pick-up point on the transmission line and an associated wave shaping circuit. The comparator controls the auxiliary variolossers in a similar manner to the way it controls the associated main variolossers or voice switches on the transmission lines thereby eliminate the malfunction described above.
However, the combination of main and auxiliary variolossers makes it difficult or impossible to interrupt a message from a first station and to insert a new message from a second station to be transmitted to the first station.
Without the strong acoustic coupling between a loudspeaker and a microphone, there would be nothing to it. Therefore, the above described disadvantage in the prior art system is another consequence derived from the inevitable existance of a strong acoustic coupling.
Disclosed in the Japanese Patent Publication No. 22281/1971 is a voice switching control system which also controls a pair of voice switches in transmission lines by sensing the outputs of microphones as described above. However, instead of using a pair of auxiliary variolossers as described above, the system employs four sensed signal amplifiers, two of which have different gains from the other two, disposed in sensed signal branches, and a pair of comparators whose outputs are connected to a second-stage comparator for controlling the voice switches, the second-stage comparator being comprised of a pair of cross-coupled NAND gates. This arrangement ensures that the first and second voice switches are operated in an exclusive `OR` (complementary) manner. However, while a talker at a first station is "continuously or incessantly" speaking to the first microphone, it is very difficult for an other person at a second station to interrupt the speech and to insert a new message which he wishes to transmit to the first station. As a conclusion, this system also has a defect similar to that of the first-mentioned prior art system.
Disclosed in a paper entitled "On the study of Voice Switching Control" by J. Takahashi et al, in the Proceedings of the Acoustic Engineering Society of Japan, pages 485-486, October 1973, is a voice switching control system which senses the levels of voice signals at four points, namely two sets of a pair of outputs and inputs, from and to the microphones and the loudspeakers of the first and second stations.
The input to the loudspeaker and the output from the microphone of each station are compared by a comparator. The pair of output signals obtained from the two comparators are applied to a logic circuit having three states or positions and which controls a pair of variolossers. A first position defines an "ON or no attenuation state"; a third position defines an "OFF or full attenuation state" and a second or intermediate position defines a "some attenuation (half the full attenuation) state".
More specifically, the comparator provides a high level signal to the logic circuit only when the level of the output from the microphone (first input to the comparator) is greater than the level of the input to the loudspeaker (second input to the comparator) by a predetermined value, and otherwise produces a low level signal. While the first comparator for comparing the microphone output and the loudspeaker input of the first station is producing a high level signal and the second comparator for comparing the microphone output and loud-speaker input of the second station is producing a low level signal, (namely, while only the user at the first station speaks to the microphone of the first station), the logic circuit controls the first variolosser to be placed in its `first (no attenuation) position` to transmit a voice message from the first station to the second, and controls the second variolosser to be placed in its `third (full attenuation) position`. While both the first and second comparators are producing high level signals (namely, while both the users at first and second stations speak to the microphones at an appropriate sound level), the logic circuit controls both of the first and second variolossers to be placed in their `second (half the full attenuation) position`.
Such arrangement and control makes it easy to insert a new voice message by the user at one station while the other user at the other station is transmitting a message because the comparator is arranged to compare the output from the microphone with the input to the loudspeaker of the same station.
However, the inserted new voice message is transmitted to the other station with a relatively restricted level of sound during the transient, simultaneous or full duplex voice communication mode of operation, because both of the variolossers introduce half the full attenuation in the voice signals in this mode. Furthermore, a closed loop for voice signals is formed. In order to avoid howling, the overall gain for the closed loop must be less than the value of gain defining a critical oscillation condition. These restrictions on the amplitude of voice signals during such transient, simultaneous voice communication make it difficult to obtain a sufficiently high level of sound at the output of a loudspeaker. Consequently, the listener often cannot perceive or hear the voice information clearly.
A delay element comprising a waveshaping circuit is interposed between the voice signal pick-up point and the control input to the comparator. The delay element delays the switching of a variolosser. Accordingly, the beginning portion of a voice message is not transmitted from one station to the other.
In general, it is desirable that voice switching control system for controlling loudspeaker type stations satisfy the following requirements:
(1) no howling,
(2) a high level of sound at the output of the loudspeaker of the station,
(3) easy to interrupt the voice message from the other station and to insert and transmit a new message from one station to the other, and
(4) little or no risk of malfunction due to ambient noises around the station.
However, each of the above requirements conflicts with one or more of the others to some extent as discussed above. All the prior art systems represent a compromise between these rather conflicting requirements, and leave room for improvement.
It is highly desirable that the voice switching control system be applicable to a telephone conference communication system including at least three loudspeaker type stations by which a conference is held.